Mouthguard
NOCSAE-stamped boil-and-bite is the right entry point. Five to fifteen dollars.
A high-top fit-test: the kid bites it after boiling, the impression should fully cover the upper teeth and gums.
Replace it every season. Mouthguards harden and crack over time.
Cleats
Soccer cleats work. Football cleats work. Rubber-stud, not metal.
NFL Flag rules require non-metal cleats. Most leagues match this rule.
Gloves
Optional. The right glove is sticky receiver gloves: small for kids 5-9, medium for 10-13, large for 14+.
A kid who hates dropping the ball will love these. A kid who plays line will not need them.